Were you taught this math in your entry level CCR class?

[Frontpointer1000]

Fascinating discussion.
I’m no CCR diver, just doing my research.

Love the math, interesting discussion on cell limiting and linearity checks. Given my naïve understanding of cell limiting, I do get concerned about assuming linearity of cells beyond 1.0 if you’re not actually checking beyond that set point. My understanding is that if a cell is non linear, then it mathematically MAY curve. I’d want to know where that curve is.

TDI has a small essay on this topic written by Jon Kieren, with contributions by Randy Thornton and Mathew Partridge. These are myths:

Scroll down to “Diluent flush will adequately validate a cell error”

Divers Deserve the Truth About Rebreathers - SDI | TDI | ERDI

 

[JohnnyC]

Fascinating discussion.
I’m no CCR diver, just doing my research.

Love the math, interesting discussion on cell limiting and linearity checks. Given my naïve understanding of cell limiting, I do get concerned about assuming linearity of cells beyond 1.0 if you’re not actually checking beyond that set point. My understanding is that if a cell is non linear, then it mathematically MAY curve. I’d want to know where that curve is.

TDI has a small essay on this topic written by Jon Kieren, with contributions by Randy Thornton and Mathew Partridge. These are myths:

Scroll down to “Diluent flush will adequately validate a cell error”

Divers Deserve the Truth About Rebreathers - SDI | TDI | ERDI

Checking beyond 1.0 is best practice and should be a requirement, I do it every dive, spiking to 1.8. That being said, we're really only interested in linearity up to 1.6ish since we're not breathing a loop with a PO2 that's any higher. I don't care where the curve goes if it's beyond the range that I will use it. I don't need to know how my miles per gallon my truck gets at 120mph since I never drive it that fast. But it is useful to know what it gets in city driving, and highway driving at 65 vs 85.

I do the same thing as @The Chairman, I isolate my ADV, use an O2 purged loop down to 8m, make sure my cells are linear through the descent and spike appropriately beyond what I will be using, then open up my ADV to add loop volume and lower my PO2 to something that's acceptable for the descent. That's with my SF2. It's a little trickier on my Pelagian because I can't isolate my ADV, so I start out with higher than min-loop on an O2 purged loop and just add O2 to 8m for volume, then blow out the lungs and either run the ADV or hit the MAV to bring PO2 down and loop volume to where it needs to be.

Being able to test PO2 at higher pressure is beneficial because you are not locked into using O2 to spike your cells (a potential safety issue as well as a cell health issue), and you can test your cells at higher atmospheric pressure while maintaining useful PO2 data. Pushing air to a PO2 above 1.6 is useful because you still benefit from seeing cell linearity within the useful range, but you're also seeing cell behavior and response at the atmospheric pressures at which you will be subjecting the cells.

 

[tbone1004]

@Frontpointer1000 one of the important parts to think about is what the real critical accuracy and precision limits of these readings really are. Does it matter if its 1.5 vs 1.6? No, not really. Deco isn't going to be significantly different, and risk of oxtox isn't going to be significantly different.
Does it matter if it's 1.4 or 1.7? Yeah that starts to get to be a bit more concerning.

@JohnnyC mentioned doing a high ppO2 check. What that is really doing is checking to make sure the cells aren't limited vs. doing a true linearity check. Again, you're only ever going to get so good of a flush on your way down, your lungs are still offgasing into the loop, etc etc. You want to see 1.6 at 20ft, but if you don't see it until 25, it's probably just an incomplete flush. I have my loop full of O2 when I start descent, and then I will do an O2 flush again when I leave my 30ft stop on the way to my 20ft stop and want to see something over 1.6. This is all really just validation of what I've seen in the De-Ox though. I'm looking more for limiting than linearity.

 

[JohnnyC]

@Frontpointer1000 one of the important parts to think about is what the real critical accuracy and precision limits of these readings really are. Does it matter if its 1.5 vs 1.6? No, not really. Deco isn't going to be significantly different, and risk of oxtox isn't going to be significantly different.
Does it matter if it's 1.4 or 1.7? Yeah that starts to get to be a bit more concerning.

@JohnnyC mentioned doing a high ppO2 check. What that is really doing is checking to make sure the cells aren't limited vs. doing a true linearity check. Again, you're only ever going to get so good of a flush on your way down, your lungs are still offgasing into the loop, etc etc. You want to see 1.6 at 20ft, but if you don't see it until 25, it's probably just an incomplete flush. I have my loop full of O2 when I start descent, and then I will do an O2 flush again when I leave my 30ft stop on the way to my 20ft stop and want to see something over 1.6. This is all really just validation of what I've seen in the De-Ox though. I'm looking more for limiting than linearity.

Good distinction. The PO2 spike is checking for current limited cells, not necessarily linearity, that's done in the pressure pot/cell checker/De-Ox. We want to make sure our cells are not current limited, and linearity doesn't matter above the range that we're looking for, but it is important that cells are linear within the range we need.

As an aside, I updated the little cheat sheet with an interactive set of cells that allows you to calculate exact expected cell readings across output ranges and depths. It's in metric. If there's interest, I'll do a set in imperial. I just don't dive that way so I did it in metric first.

 

[tbone1004]

Good distinction. The PO2 spike is checking for current limited cells, not necessarily linearity, that's done in the pressure pot/cell checker/De-Ox.

I think this was probably said earlier and you obviously know it, but for @Frontpointer1000 to understand since this concept annoyingly escapes most CCR divers and instructors. If you actually understand how this works, this is the thought process you go through.

Say I check my cells and they all test 90% linear. I have single point calibration which is done at 1.0
When I go to 6m/20ft, I "know" that I am breathing a ppO2 of 1.6, but I expect to see the handsets say 1.44

If they *one, two, all, whatever* say 1.3, then I will do another O2 flush because the most common issue there would be an incomplete flush. If they still say 1.3 after the O2 flush, then they are most likely current limited. Means that they will not go over 1.3ish regardless of how much O2 is actually in the loop. Think a clogged air filter in your house where the air flow is restricted no matter how hard the fan blows.

Couple of options. First, most prudent, and what you arguably should do is call the dive, pop a new cell in, wait a couple of hours for it to wake up, and sort yourself out and try again later that day or tomorrow.
Other option.
You know that the cells check out at 1.0, and can choose to run the dive at 1.0. I run my setpoint at 1.1 because of my HUD, so I know that when I tell it that I'm diving 1.1, I'd actually be diving 1.22. Again, not going to bother me for Ox-Tox either immediate or duration based, and it makes my deco more conservative so no harm no foul. When you can get your Dil to a ppO2 of 1.1ish *say 32% at 85ft*, then you do a full dil flush. You know that that should be around 1.1, and if it is, you can make the choice to do the whole dive at 1.1.
Downside of this, is you are going to have more deco if you planned a deco ppO2 of 1.3/1.4. If doing trimix dives, you also can't have a higher deco ppO2 until you hit your 20ft stop because once you hit that 1.2-1.3 limit, you have no idea what the actual ppO2 you are breathing is.
Once back at 20ft, you can do an O2 flush. Deco may still think 1.3 so you'll have a longer deco obligation if using the onboard computer as primary deco monitor, but that isn't a bad thing. You'll be breathing somewhere under 1.6 once you're at 20ft, so you won't oxtox.

 

[The Chairman]

Again, you're only ever going to get so good of a flush on your way down, your lungs are still offgasing into the loop, etc etc.

If you start with a %100 on the surface, then at least on an SF2, the flush is spot on. With your head up 6 inches, and you smile you automatically loose %90 of your loop volume. The single counter lung collapses and evacuates so you only need a small burst to push the rest out of your loop and saturate the unit with oxygen. If I don't see 1.6 PPO2, I'm not proceeding. Then a simple slide of the Adv valve and another smile, and the resultant purge of air will clean it out. Just remember that you'll have a different PPO2 if you're using NitrOx rather than air for your dil.

 

[Frontpointer1000]

@tbone1004
@JohnnyC
@The Chairman
Awesome comments and points of clarification!
I agree that from a mathematical model, understanding linearity and where that line may begin to curve may not matter if it is outside of her useful (practical) range.

reading these comments and putting that with Jill Heinerth’s book really begins to solidify this issue for me. Although I still see CCR as down the road for me, Your comments helped solidify the foundation upon which I’m building.

Thx!!

 

[broncobowsher]

OK, a little late to this. The exact math on the first post, no wasn't taught like that. But the concept was put together.

My typical dive pattern. Run a high O2 on the surface. On decent first spike with O2 and blow past my highest planned PPO2 (1.5, 1.6, anything too high). Then as I descend chase that number down with Dil. Sometimes I might have to burp a little out of the loop for a little extra Dil if I can't chase the number down enough. No exact science, but I start with cells that are capable of spanning more range then I plan to use.

I also do rotating cells. The cells are bought different times, different batches, different expiration dates. Each one should die at different times. Even if one was from a bad batch it isn't two (or more) from the same batch.

 

[tbone1004]

@broncobowsher why bother burping it down vs. just breathing it down if it doesn't exceed 1.5/1.6? It'll come down on its own in a few minutes

 

[rjack321]

Answering the poll: yes
Except I do: air mV divided by 0.209 = mV in 100%
Multiply mV in 100% x 1.2 = target mV on the dive.

Sometimes I do run a whole (shorter) dive at 1.3 but if I am forced to run my exit on mV outputs because the calibration is lost then I'm sticking to lower mV and just having the deco be a tiny bit longer.
I have had a few cells fail both high and low, but I have never had a cell less than a year old fail in a subtle way. All my cell failures have been rather dramatic and obvious. Nevertheless, I would not use a cell with 10% deviation from linearity, I expect them to be +- 5% of expected.